The control and/or elimination of undesirable impurities and by-products from various manufacturing operations has gained considerable importance in view of the potential pollution such impurities and by-products may generate. One conventional approach for eliminating or at least reducing these pollutants is by thermal oxidation. Thermal oxidation occurs when contaminated air containing sufficient oxygen is heated to a temperature high enough and for a sufficient length of time to convert the undesired compounds into harmless gases such as carbon dioxide and water vapor. Catalysts can be used to enhance the process.
One application of such oxidizers is in conjunction with web drying apparatus, including flotation dryers. Such dryers are capable of contactless supporting and drying a moving web of material, such as paper, film or other sheet material, via heated air issuing from a series of typically opposing air nozzles, requires a heat source for the heated air. As a result of the drying process, undesirable volatile organic compounds (VOCs) may evolve from the moving web of material, especially where the drying is of a coating of ink or the like on the web. Such VOCs are mandated by law to be converted to harmless gases prior to release to the environment.
In view of the high cost of the fuel necessary to generate the required heat for oxidation, it is advantageous to recover as much of the heat as possible. To that end, U.S. Pat. No. 3,870,474 discloses a thermal regenerative oxidizer comprising three regenerators, two of which are in operation at any given time while the third receives a small purge of purified air to force out any untreated or contaminated air therefrom and discharges it into a combustion chamber where the contaminants are oxidized. Upon completion of a first cycle, the flow of contaminated air is reversed through the regenerator from which the purified air was previously discharged, in order to preheat the contaminated air during passage through the regenerator prior to its introduction into the combustion chamber. In this way, heat recovery is achieved.
U.S. Pat. No. 3,895,918 discloses a thermal rotary regeneration system in which a plurality of spaced, non-parallel heat-exchange beds are disposed toward the periphery of a central, high-temperature combustion chamber. Each heat-exchange bed is filled with heat-exchanging ceramic elements. Exhaust gases from industrial processes are supplied to an inlet duct, which distributes the gases to selected heat-exchange sections depending upon whether an inlet valve to a given section is open or closed.
Current state-of-the-art regenerative thermal or catalytic oxidizers that utilize two heat exchange beds generally include the addition of some means of capturing the untreated gases which bypass the oxidizer during valve switches or cycle changes. These systems are referred to as entrapment chambers or puff chambers, an example of which is shown in U.S. Pat. No. 5,833,938, the disclosure of which is hereby incorporated by reference. Use of such chambers increases the VOC destruction efficiency of the apparatus, since untreated gases that otherwise would escape are captured and recycled for treatment. Such chambers typically employ one or more valves, such as poppet valves or butterfly valves, to divert the untreated gases into a holding plenum during the cycle change, and then bleed or purge the chamber clean between cycle changes. The plenum exhaust may be directed back to the system inlet or directly to the burner chamber to be treated.
Problems with current systems include the cost of the damper system needed to operate the chamber, the modifications necessary to the regenerative oxidizer to accommodate the chamber, the high pressure drop that occurs across the chamber which requires upsizing the main draft fan, and the process flow disturbances that occur as a result of the switching of the diverting dampers and back pressure of the chamber.
It would be desirable to design an entrapment chamber that reduces or eliminates the aforementioned drawbacks of conventional systems.
The problems of the prior art have been overcome by the present invention, which provides a regenerative oxidizer including an entrapment chamber and damper assembly, as well as a method of oxidizing volatile organic compounds in a gas using such apparatus. To improve the VOC destruction efficiency and eliminate opacity issues resulting from heat exchange matrix regeneration, untreated fluid can be diverted away from the oxidizer exhaust stack and directed into a xe2x80x9cholding vesselxe2x80x9d or VOC entrapment chamber. The function of the entrapment chamber is to contain the slug of untreated fluid which occurs during the matrix regeneration process long enough so that the majority of it can be slowly recycled (i.e., at a very low flow rate) back to the inlet of the oxidizer for treatment, or can be supplied to the combustion blower as combustion air, or slowly bled to atmosphere through the exhaust stack. The untreated fluid in the entrapment chamber must be entirely evacuated within the time frame allotted between matrix regeneration cycles since the process must repeat itself for all subsequent matrix regenerations.
The assembly utilizes a single swing damper in order to divert the exhaust flow from the oxidizer either to exhaust or to the entrapment chamber. The entrapment chamber preferably has a modular construction, which facilitates expanding or decreasing the volume of the chamber. The design facilitates installation and can be retrofitted on existing equipment. The damper diverts the flow of gas with minimal or no negative impact on the process flow.